A protective component of the type referred to is disclosed in, for instance, the publication EP 0 715 328 B1. Applied to the surface of a substrate, which consists, for instance, of aluminium oxide, (using thin film or thick film technology) there is an electrically conductive resistive layer which is structured in the form of a rectangular resistor and is provided on two opposite sides with electrical contacts by the application of conductive metal layers. If the electrical resistor R which is manufactured in this manner, has a current I of a predetermined magnitude applied to it, the power loss P=I2×R is converted into heat at the location of the resistive or heating element. An electrically insulating layer is applied over the resistive layer and a fusible element arranged between two supply contacts is produced over the electrically insulating layer.
The fusible element consists of a conductive layer of a low melting point metal, structured in the form of a strip, whereby the layer strip is contacted at its ends with the aid of metallic conducting layers. The strip of the low melting point metal layer is arranged above the heating element so that the heat produced by the heating element is transported to the fusible element, whereby the low melting point metal melts when the temperature of the fusible element exceeds a predetermined threshold value.
For structural reasons, only a portion of the heat produced by the heating resistor is transported to the fusible element; a considerable portion of the heat produced is dissipated into the substrate and to the surroundings of the device. Both the heat dissipated to the substrate and also the heat transmitted through the electrically insulating intermediate layer to the fusible element is transported to a very large extent by thermal conduction, whereby the materials, which conduct the heat, heat up and can store a certain amount of heat. As a result of the small spatial distance of the fusible element from the heating element in the known arrangement, the heat is transported relatively rapidly by thermal conduction, whereby time delays as a result of the storage of heat is not considered in detail with the known device.
It is further known in the prior art to manufacture protective elements, particularly fuse elements, which interrupt the flow of current at predetermined currents and after predetermined times. With very high currents, the interruption should occur within the shortest possible time (quick action). In the case of fuse elements, in which the heating element and the fusible element are connected in series, the current flow is preferably interrupted by destruction of the resistive layer of the heating element. If the fuse element is subjected for a relatively long period of time to a current and this current increases only slowly, the fuse element is broken by melting and rupture of the fusible element when the current exceeds a predetermined threshold value. This minimal current, which ruptures the component only after it has been applied for a relatively long period of time, corresponds to the threshold current of the fuse element. In the case of currents through the fuse element, which have a magnitude above the threshold current of up to a few multiples of the threshold current but which flow for only a relatively short period of time, the fuse component is tripped after predetermined periods of time by rupturing of the fusible element. Such fuse components are commonly characterised by a time-current characteristic, which determines for the component after what periods of time and at what currents (suddenly applied) rupture of the component occurs.
It is the object of the invention to provide a method of producing a protective component, in which the time-current characteristic can be set to be better, particularly in the region above the threshold current up to a few multiples of the threshold current.